This invention relates to the field of electronics and particularly to the field of driver electronics for thin film electroluminescent display devices.
Electroluminescence is the emission of light from a luminescent material when an electric field of sufficient amplitude is applied to the material. This phenomena has been used to construct display panels by using the luminescent material as the dielectric in a parallel plate capacitor in which one of the conducting plates is transparent. When alternating voltages or pulses are applied to the plates, the luminescent material emits light.
Electroluminescent video display panels have been constructed by depositing conductive rows and columns on opposite, non-conductive plates of a capacitor to form an x-y matrix. The coordinates of the matrix are the pixels of the display. When a voltage differential is created between a row and a column, the luminescent material between the crossing electrodes emits light at that pixel.
Electroluminescent technology offers the potential of providing compact, flat panel displays rather than the bulky cathode ray tube displays now in wide use. Small electroluminescent display panels can be driven by integrated solid state circuits to provide miniature video systems that are not practical using cathode ray tube displays.
To realize the potential of electroluminescent displays, drive circuits are required which are inexpensive, reliable, require low power, and fully utilize the electroluminescent capacity of the display, including gray-scale image generation.
Electroluminescent displays can be gray-scale imaged because the brightness of an AC excited luminescent material increases with increasing voltage above the threshold voltage. This property is shown in FIG. 1. In this figure, threshold voltage 2 of the material is approximately 104 volts, and the brightness increases rapidly as the voltage is raised to approximately 118 volts. The brightness can be controlled for gray-scale imaging by amplitude modulation of the applied voltage.
Although the high threshold characteristic for light emission simplifies matrix-addressing an electroluminescent display, it does require the use of high voltage integrated circuits. Additionally, the steepness of the electro-optic response curve (FIG. 1) necessitates good uniformity of output voltage from different drivers for the same desired brightness level.
A prior art, low impedance row driver system which uses analog amplitude modulation for gray-scale manifestation is described in "Practical Application Technologies of Thin-Film Electroluminescent Panels" by M. Takeda, Y. Kanotani, J. Kishishita, T. Inoguchi, and K. Okano as reported in the Society for Information Display's (SID) 1981 San Diego Symposium. The high accuracy, high voltage analog drivers used in such prior art systems are expensive both in cost and in circuit real-estate. Also, an electroluminescent display appears primarily as a capacitive load, thus causing a real impedance driver to dissipate an excessive amount of power.
A prior art system which uses linear drivers is described in "Monolithic Driver Chips for Matrixed Gray-Shaded TFEL Displays" by T. Gielow, R. Holly, and D. Lanzinger, SID 1981 Symposium in New York. Such drivers waste power because the voltage drop and current through the device occur simultaneously. Each column driver will source current into the capacitance of the floating rows as an independent action relative to the current sourced to the driven row. The sourcing (or sinking) of current to the set of nonselected rows is the major contributor to the inefficiency of such prior art drive schemes.